Office for Mac started before Office was Office, i.e. 1984 with word and later on powerpoint. Apple was the king and Microsoft had no choice but to support them. It also had virtually had no viable competition and as of today remains the dominant office suite on Mac. Although iWorks is making some inroads, Microsoft will continue to have a very large chunk of the Mac office productivity market for the foreseeable future.

Windows Phone on the other hand is new so it makes a lot of sense to initially avoid a relatively tiny Mac/Linux market sector which is also mostly hostile towards Microsoft.

If Windows Phone becomes popular and proves successful and Microsoft captures as much of the 90% of the Windows market as it could, then it would make business sense to go after the remaining 10%, i.e. the niche. Before that time comes, the overhead cost is not worth it.